The present invention relates to a heat dissipating component capable of managing the heat from a heat source such as an electronic device. More particularly, the present invention relates to a heat dissipating component effective for dissipating the heat generated by an electronic device, wherein the heat dissipating component is constructed by assembling together an anisotropic graphite planar element with a high thermal conductivity core element.
With the development of more and more sophisticated electronic devices, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates the negative effects of excessive heat.
With the increased need for heat dissipation from microelectronic devices, thermal management becomes an increasingly important element of the design of electronic products. Both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an exponential increase in the reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance.
Several types of heat dissipating components are utilized to facilitate heat dissipation from electronic devices. The present invention is directly applicable to several of these heat dissipating components, including those generally referred to as heat spreaders, those generally referred to as cold plates, and those generally referred to as heat sinks, among others.
These heat dissipating components facilitate heat dissipation from the surface of a heat source, such as a heat-generating electronic device, to a cooler environment, usually air. In many typical situations, heat transfer between the solid surface of the electronic device and the air is the least efficient within the system, and the solid-air interface thus represents the greatest barrier for heat dissipation. The heat dissipating components seek to increase the heat transfer efficiency between the electronic device and the ambient air primarily by increasing the surface area that is in direct contact with the air or other heat transfer media. This allows more heat to be dissipated and thus lowers the electronic device operating temperature. The primary purpose of a heat dissipating component is to help maintain the device temperature below the maximum allowable temperature specified by its designer/manufacturer.
Typically, the heat dissipating components are formed of a metal, especially copper or aluminum, due to the ability of metals like copper to readily absorb heat and transfer it about its entire structure. In the case of heat sinks, copper heat sinks are often formed with fins or other structures to increase the surface area of the heat sink, with air being forced across or through the fins (such as by a fan) to effect heat dissipation from the electronic component, through the copper heat sink and then to the air.
Limitations exist, however, with the use of metallic heat dissipating components. One limitation relates to the relative isotropy of a metal that is, the tendency of a metallic structure to distribute heat relatively evenly about the structure. The isotropy of a metal means that heat transmitted to a metallic heat dissipating component becomes distributed about the structure rather than being preferentially directed to a desired location.
In addition, the use of copper or aluminum heat dissipating elements can present a problem because of the weight of the metal, particularly when the heat transmitting area of the heat dissipating component is significantly larger than that of the electronic device. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cm3) and pure aluminum weighs 2.70 g/cm3 (compare with graphite articles, which typically weigh less than about 1.8 g/cm3).
For example, in many applications, several heat sinks need to be arrayed on, e.g., a circuit board to dissipate heat from a variety of components on the board. If metallic heat sinks are employed, the sheer weight of the metal on the board can increase the chances of the board cracking or of other equally undesirable effects, and increases the weight of the component itself.
In the case of larger heat dissipating components such as for example that class of components known as heat spreaders, the weight of a pure copper heat spreader requires special mechanical features and designs to hold the heat spreader.
What is desired, therefore, is a heat dissipating component effective for dissipating heat from a heat source such as an electronic device. The heat dissipating component should advantageously be relatively anisotropic, as compared to a metal like copper or aluminum and exhibit a relatively high ratio of thermal conductivity to weight. One group of materials suitable for use in heat sinks are those materials generally known as graphites, but in particular anisotropic graphites such as those based on natural graphites and flexible graphite as described below.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cm3 to about 2.0 g/cm3. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
The present invention provides a thermal management system which includes an anisotropic planar element having a relatively high thermal conductivity in the plane of the planar element and having a relatively low thermal conductivity across a thickness of the planar element in a direction normal to the plane of the planar element. The planar element has a cavity therein, and a core or insert is closely received in the cavity. The core in this embodiment is constructed of an isotropic core material so that heat from a heat source can be conducted via the core into the thickness of the planar element and then out across the plane of the planar element
In another embodiment the present invention provides a thermal management system which includes a heat source having a heat transmitting surface, an anisotropic graphic planar element, and an insert. The planar element has x and y dimensions defining a generally planar extent of the planar element and has a z dimension defining a thickness of the planar element. The planar element has a relatively high thermal conductivity in the x and y directions and a relatively low thermal conductivity in the z direction. Thus the x and y directions as used in this disclosure correspond to what are conventionally referred to as the xe2x80x9caxe2x80x9d axes for anisotropic graphite, and the z direction as used herein corresponds to the xe2x80x9ccxe2x80x9d direction or axis of anisotropic graphite. The planar element has a cavity defined therein extending at least partially through the thickness of the planar element. The insert is received in the cavity in heat conducting engagement with the planar element. The insert has a heat receiving surface engaging the heat conducting surface of the heat source, so that heat from the heat source flows across the heat transmitting surface and the heat receiving surface into the insert in the z direction and then out through the planar element in the x and y directions.
In another embodiment of the invention a method is provided for dissipating heat from the heat source. The method includes steps of:
(a) providing an anisotropic heat dissipating element capable of relatively high conductivity in the x and y directions, and having relatively low thermal conductivity in a z direction perpendicular to the x and y directions, the heat dissipating element having a cavity defined therethrough in the z direction and having an isotropic heat conducting insert disposed in the cavity;
(b) placing the insert in heat conducting engagement with a heat source;
(c) conducting heat from the heat source through the insert and into the anisotropic heat dissipating element; and
(d) conducting heat through the heat dissipating element in the x and y directions.
Accordingly, it is an object of the present invention to provide improved designs for heat dissipating components including anisotropic graphite planar members.
Another object of the present invention is the provision of a heat dissipating component including a high thermal conductivity core for conducting heat from a heat source to an anisotropic graphite heat dissipating element.
And another object of the present invention is the provision of heat dissipating components of relatively light weight such as provided by graphite, but having a high thermal conductivity at the interface of the heat dissipating component with the heat source.
And another object of the present invention is the provision of composite heat dissipating components utilizing anisotropic graphite material to conduct heat across the major surface areas of the component, while using isotropic high thermal conductivity materials such as copper for conducting heat from the heat source into the body of the anisotropic materials.
And another object of the present invention is the provision of economical constructions for heat dissipating components.
Other and further objects, features, and advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.